Such a hydraulic control arrangement having two metering pumps in the metering pump unit is known from U.S. Pat. No. 5,806,561 B. In a “normal” steering mode both metering pumps are connected hydraulically in parallel and are operated mechanically in parallel. In this mode of operation the metering pump unit has a rather large displacement. When the control arrangement is used as a steering unit in a vehicle, a small steering angle at the steering wheel causes a rather large steering angle at the steered wheels. In an “emergency” mode of operation it is possible to short circuit one steering pump so that only the remaining steering pump can work. In this mode of operation the torque produced by a driver to rotate the steering wheel can be reduced. However, a greater number of revolutions are necessary to get the desired steering angle of the steered wheels. The control valve can be used to switch between the two modes of operation even if there is no emergency case. The control valve can for example be controlled in dependency on speed or by the driver if he wishes to obtain more sensitive steering.
When the control valve is switched in normal mode the pump pressure has to exceed the force of a spring so that a valve element of the control valve can be moved in a position connecting the two metering pumps.
In hydraulic systems having a higher operating pressure it is in some cases difficult to get the desired switching behavior of the shut-off valve. For example, in some cases the shut-off valve should be switched to a state in which both metering pumps are connected in parallel only when the pump pressure exceeds 40-70 bar. Below this pressure level only one metering pump should meter out oil to the working connection.